How do I know what rear axle I have?

The quickest way to know your rear axle is to read the factory markings: check the data plate or door sticker for the axle ratio, inspect the differential housing and any tags for the gear code, and verify with the vehicle’s VIN decoded through a dealer or online resource.

Where to look on the vehicle

These are the most reliable places to find axle information:

• Vehicle data plate or manufacturer's sticker: Usually located in the driver’s door jamb or inside the glove box area. Look for entries labeled “Rear Axle,” “Axle Ratio,” or a numeric ratio (for example 3.55, 4.10).


• Differential cover markings: Remove the cover only if you’re comfortable, and read stamped numbers on the ring gear or carrier that indicate the gear ratio and, occasionally, the differential type.


• Axle tag or stamping on the axle tube: Some makes place a small metal tag or a stamped code near the brake line bracket or shock mount with a code that maps to a specific ratio or configuration.


• VIN and build sheet or window sticker: The original documentation often lists the rear axle ratio and differential option for the vehicle configuration.


• Online portals and maintenance manuals: Many manufacturers provide online portals or PDFs that reference the axle code or option codes for your exact model year and trim.

If you can’t locate markings, a dealer service department or a qualified mechanic can pull the axle code from factory databases or measure the gear ratio directly.

How to decode and confirm the exact axle

Use VIN information and on-vehicle markings to confirm the precise axle configuration. Here are reliable steps:

• VIN decoding: Enter the vehicle’s VIN into an official manufacturer decoder or a reputable third-party tool to reveal the axle option codes, gear ratio, and differential type for your exact build.


• Check the axle code on data plates: Many cars list a code that corresponds to a gear ratio and differential type. Cross-reference this code with the manufacturer’s code tables.


• Compare with the window sticker/build sheet: The original documents should specify the axle ratio and whether an LSD or other differential option was installed.


• Determine ratio without disassembly when necessary: If markings are unreadable, a shop can measure the gear ratio by rotating the driveshaft a known number of turns or by counting teeth on the ring gear after removing the cover.

By combining VIN decoding with on-vehicle markings, you can determine the exact rear axle with high confidence. When in doubt, consult a dealership or a trusted mechanic who can pull the correct factory configuration for your vehicle.

What you can learn about your axle once identified

Knowing the axle details helps with maintenance, repairs, and upgrades. Common information includes:

• Gear ratio: Examples include 2.73, 3.08, 3.55, 4.10, which influence acceleration and highway fuel economy.


• Differential type: Open diff, limited-slip (LSD), locking differential, or other variants. LSD is common on performance or some trims, while open diffs are typical on base models.


• Drive layout and suspension: Whether the vehicle uses a solid rear axle or independent rear suspension (IRS) affects part compatibility and service procedures.


• Parts compatibility and service considerations: Brake components, seals, studs, and axle shafts must match the exact axle configuration when ordering parts or fluids.

Differences between solid rear axle and independent rear suspension

Solid rear axles transmit power through a single housing with a rigid axle to both wheels, often found on trucks and some performance models. Independent rear suspension uses separate suspension links for each wheel, improving ride quality and handling. The identification usually comes from the overall chassis design and the presence of separate half-shafts or a more complex differential mounting.

Summary

To identify your rear axle, begin with the factory data plate or door sticker, then inspect the differential housing and any tags for a gear ratio or code. Verify with the VIN through official decode tools and compare against the window sticker or build sheet. Knowing the exact axle ratio, differential type, and whether the setup is a solid axle or IRS helps with maintenance, parts selection, and future upgrades. If markings are unclear, a dealership or qualified technician can pull the official configuration for your vehicle.

How do I identify my rear axle?

To identify your rear axle, first look for an ID tag or stamped numbers on the axle housing, which may indicate the model, gear ratio, and other specs. If tags are missing, use visual identification by counting the bolts on the differential cover, noting the cover's shape, and measuring the ring gear diameter and bolt pattern. You can also check for stamped numbers on the axle tubes or on the differential cover itself, often near the fill plug. 
Using ID tags and stamps 

• Find the tag: Look for a metal tag bolted to the differential cover or a stamp on the axle tube. Trailer axles often have a sticker on the back or side. 
• Decode the information: The tag or stamp may include codes for the gear ratio, whether it has a limited-slip differential, and the axle's manufacturing plant and date. 
• For Ford axles: Look for the tag on the differential cover. Numbers on this tag often contain information about the gear ratio and type, with specific codes for different model years, says HOT ROD Network. 
• For trailer axles: Labels are often on the back of the axle and include the serial number, capacity, and build details, according to Dexter Group. 
• For GM axles: Numbers may be stamped on the axle tubes and you can decode them using a chart. The ratio may also be stamped on the ring gear after removing the cover, with the format [pinion teeth]-[ring gear teeth], notes Speedway Motors. 

This video explains how to read a Dexter axle label: 47sDEXTER TVYouTube · Aug 22, 2022
Using visual identification
You can watch this video to see a visual comparison of different rear axles: 28sJeep Trails & Barbells YouTube · Nov 23, 2022

• Differential cover bolts: Count the number of bolts that hold the differential cover on. Different axle models have different bolt patterns, according to Sierra Gear & Axle. 
• Cover shape: Note the shape of the differential cover, as it can be a key identifier. For example, some Dana axles have covers that look identical to their front-wheel-drive counterparts, explains this YouTube video. 
• Ring gear diameter: Measure the diameter of the ring gear. You may need to remove the cover to do this, says Sierra Gear & Axle. 
• Axle tube shape: Check if the axle tubes are a consistent diameter all the way out or if they taper towards the ends. For example, a 7.5-inch axle has a consistent diameter, while an 8.5-inch has tapered tubes, as shown in this Facebook post. 

How do you tell if you have a 7.5 or 8.5 rear end?

You can tell the difference between a 7.5 and 8.5 rear end by looking at the shape of the cover, the shape of the axle tubes, and the protrusions on the bottom of the housing. The 8.5 has a round cover with larger, rectangular protrusions at the bottom, while the 7.5 has a more squared or oval cover with smaller, crescent-shaped lugs at the bottom. Additionally, the axle tubes on an 8.5 are flared or stepped down as they enter the pumpkin, while the 7.5 has straight axle tubes.
 
This video shows how to visually identify a GM 10 bolt rear axle, including how to distinguish between a 7.5 and 8.5 rear end: 53sandhotrodshopYouTube · Dec 14, 2023
Visual differences

• Cover shape: An 8.5 has a round rear cover, whereas the 7.5 has a more squared-off or oval cover. 
• Axle tubes: The tubes on an 8.5 will appear to swell or flare out just before entering the differential housing. The 7.5 has straight axle tubes. 
• Bottom protrusions: The 8.5 has larger, rectangular "ears" or protrusions at the bottom of the housing (around the 5 and 7 o'clock positions). The 7.5's protrusions are smaller and more crescent-shaped. 

Other methods

• Measure the bolts: On some GM 10-bolt rears, the distance between the bottom center bolt and the nearest bolt on either side can be used for identification. 
  • 8.5: The distance is approximately 3.753.753.75 inches. 
  • 7.5: The distance is approximately 3.253.253.25 inches. 
• Check the vehicle's RPO codes: Look for a sticker in the driver's side doorjamb or glove box. The codes listed under "Axle" or "RPO" will specify the factory-installed rear end. This is the most accurate method if the rear end is original. 

How do I know if I have a 3.73 or 4.10 gear?

And since we live in the day and age of 2025. Uh if you're curious. You can easily. Google you know uh the number of teeth on a ring gear.

Does the VIN number tell the axle ratio?

Yes if it's not been molested. If you bought it used and there is any indicator that the covers have been removed from the truck you need to pull them and visually verify the ring and pinion for the correct gear ratio.